Large molybdenum isotope variations trace subsurface fluid migration along the Dead Sea transform

Abstract

Large (−0.6‰ to +2.3‰) δ 97/95 Mo variations occur in molybdenum-enriched iron oxide veins located in anticlines adjacent to the Dead Sea transform, and are thought to refl ect the solutions from which the veins formed because of effi cient scavenging of Mo by iron oxides. The wide range of δ 97/95 Mo values occurs in a paleohydrologic setting in which dense, concentrated, evaporitic marine brines in the Dead Sea Rift valley (δ 97/95 Mo ~1.6‰) penetrated and interacted with sedimentary rocks of the rift fl anks, which included Cretaceous bituminous chalks (δ 97/95 Mo 0.8‰-1.2‰), before intermixing with Febearing groundwater containing molybdenum of igneous and clastic origin. The δ 97/95 Mo values higher than seawater (1.6‰-2.3‰) are attributed to Rayleigh isotopic fractionation along the path of the brine migration. Molybdenum isotopes thus reveal a powerful potential to trace the sources and processes governing subsurface metalliferous fl uid migration.

Application of radon and radium isotopes to groundwater flow dynamics: An example from the Dead Sea

“…Interestingly, the freshwater end member of the Dead Sea groundwater contains significantly higher activities of 222 Rn and 226 Ra (~1000-3000 and~5-8 dpm/L, respectively, sample yk14 and Ein Shulamit, Appendix A) compared with that in the groundwater from the same aquifer (Judea) further away from the rift valley ( 222 Rn = 800 and 226 Ra = 0.16 dpm/L, Moise et al, 2000). This may be because of the enhanced presence of oxides in the carbonate rocks of the Judea Group near the Dead Sea due to the interaction with the Dead Sea brine (Starinsky, 1974;Ryb et al, 2009). The oxides are uranium-rich (Ilani et al, 1988), which might be the source for both the increased 226 Ra and 222 Rn in the local groundwater.…”

16 Good Golly, Why Moly? THE STABLE ISOTOPE GEOCHEMISTRY OF MOLYBDENUM

“…Molybdenum isotope studies point to the importance of redox reactions on the δ 98 Mo of Mobearing mineral phases in low-temperature systems (Ryb et al, 2009;Greber et al, 2011;Song et al, 2011). In a Pliocene low-temperature system (100-160°C) in Switzerland, molybdate may have been transported by oxidizing surface waters into brecciated rocks (Grimsel breccia) where it was reduced, leading to precipitation of Mo-bearing sulfide phases (Greber et al, 2011; the mineralogy could not be identified by the authors).…”

“…A study of Mo-rich iron oxide veins by Ryb et al (2009) revealed significant Mo isotopic variation of greater than 4‰ in a low temperature mineralizing system associated with the Dead Sea transform. The isotopic variation likely reflects interaction of dense evaporitic marine brines (δ 98 Mo ~ 2.3‰) with isotopically lighter igneous and sedimentary rocks, as well as Rayleigh distillation of Mo isotopes along the brine flow path.…”

Fluid evolution in fracturing black shales, Appalachian Basin

“…Nearly universal trends in sediment permeability with depth, as well as diagenetic cements with host-rock lithology, suggest that subsurface fluid-flow is limited and relatively closed-system fluid behavior dominates (Bjørlykke, 2014). Most studies documenting significant fluid flow through the subsurface invoke enhanced permeability resulting from brittle deformation (Gleeson et al, 2003;Barker et al, 2006;Ryb et al, 2009;Vilasi et al, 2009;Dewever et al, 2013;Haeri-Ardakani et al, 2013;Wazir et al, 2014;Cai et al, 2015;Jung et al, 2015;Worden et al, 2015). However, the presence of fractures does not necessitate that fluid flow has occurred, because it has long been appreciated that fractures may open and fill in a chemically closed system with little or no fluid transport (Dietrich et al, 1983;Czerniakowski et al, 1984;Banks et al, 1991;Gao et al, 1992;Lacazette and Engelder, 1992;Henry et al, 1996;Wiltschko and Morse, 2001;Blyth et al, 2004;Wangen and Munz, 2004;Verlaguet et al, 2011).…”